Monolithically integrated photonic circuits are useful as optical data links in applications such as, but not limited to, optical communications, high performance computing, and data centers. For mobile computing platforms too, a PIC is a useful means of I/O for rapidly updating or syncing a mobile device with a host device and/or cloud service where a wireless link has insufficient bandwidth. Such optical links utilize an optical I/O interface that includes an optical transmitter and an optical receiver. One challenge with the optical I/O interface is coupling light between monolithically integrated photonic circuits, which are fabricated on a micrometer scale, and separately packaged components (e.g., optical fiber, etc.) which are assembled on the millimeter scale. A PIC may utilize vertical or edge-based optical I/O coupling techniques. The edge coupling technologies have a substantial drawback in that device testing requires an edge to be provided, typically by singulating the substrate upon which the PIC was fabricated into individual PIC chips. The vertical coupling technologies, while advantageously amenable to “wafer-level” PIC testing, typically have lower coupling efficiency than do the edge technologies and may further suffer from polarization issues and wavelength dependence, as a function of the implementation.
Light travelling horizontally a waveguide is technically difficult to reflect with mirror surfaces because the redirected light tends to be off-normal (e.g., a (111) plane in (100) silicon forms a mirror facet at 54.7° from a (100) plane, or 19.4° off-normal). For such circumstances, the presence of a material having a sufficiently high refractive index is needed in the path of the redirected light to avoid total internal reflection (TIR) of the redirected light at the interface of the waveguide. For at least this reason, actually coupling light vertically out of (or into) a horizontally oriented waveguide in the absence of such high index materials poses technical difficulty.
A vertical coupling technique that offers vertically directed light coupling for a horizontally oriented waveguide having high efficiency would be advantageous in the provision of PICs, such as optical receivers and/or transmitters.